At present, automotive parts, e.g., gears, transmissions, and seat recliners, have been produced by processing hot rolled steel sheets, which are carbon steels for machine structural use specified in JIS G 4051, through cold working into predetermined shapes and applying a hardening treatment to ensure a predetermined hardness. Consequently, a mother hot rolled steel sheet is required to have excellent cold workability and hardenability, and various steel sheets have been proposed previously.
For example, Patent Literature 1 discloses a hot rolled steel sheet having excellent toughness after hardening and containing, as steel components, C: 0.10% to 0.37%, Si: 1% or less, Mn: 1.4% or less, P: 0.1% or less, S: 0.03% or less, sol. Al: 0.01% to 0.1%, N: 0.0005% to 0.0050%, Ti: 0.005% to 0.05%, B: 0.0003% to 0.0050%, and the remainder composed of Fe and incidental impurities, on a percent by mass basis, while B−(10.8/14)N*≥0.0005% and N*=N−(14/48)Ti, where N*=0 when the right side ≤0, are satisfied, wherein the average grain size of TiN which is a precipitate in the steel is 0.06 to 0.30 μm and a prior austenite grain size after hardening is 2 to 25 μm.
Patent Literature 2 discloses a method for manufacturing a Ti-B base high carbon steel sheet without tempering. The steel sheet has excellent cold workability, hardenability, and toughness after a heat treatment and contains C: 0.15% to 0.40%, Si: 0.35% or less, Mn: 0.6% to 1.50%, P: 0.03% or less, S: 0.020% or less, sol. Al: 0.01% to 0.20%, N: 0.0020% to 0.012%, Ti: 0.005% to 0.1%, B: 0.0003% to 0.0030%, and the remainder composed of Fe and incidental impurities, on a percent by mass basis, where B 0.0032−0.014×sol. Al−0.029×Ti is satisfied.
Patent Literature 3 discloses a high carbon hot rolled steel sheet having excellent cold workability, a composition containing C: 0.20% to 0.48%, Si: 0.1% or less, Mn: 0.20% to 0.60%, P: 0.02% or less, S: 0.01% or less, sol. Al: 0.1% or less, N: 0.005% or less, Ti: 0.005% to 0.05%, B: 0.0005% to 0.003%, Cr: 0.05% to 0.3%, and the remainder composed of Fe and incidental impurities, on a percent by mass basis, where Ti−(48/14)N≥0.005 is satisfied, and a microstructure in which a ferrite average grain size is 6 μm or less, carbide average grain size is 0.1 μm or more and less than 1.20 μm, and a volume fraction of ferrite grains substantially not containing carbide is 5% or less.